1. Field of the Invention
The present invention relates generally to fluid-powered motors, and in particular to a valve for automatically reversing a hydraulic, piston-and-cylinder unit.
2. Description of the Prior Art
Fluid-powered devices are well known and a variety of different designs have been devised to meet the requirements of particular applications. These include hydraulic and pneumatic piston-and-cylinder units which are connected to pumps and compressors. Piston-and-cylinder units, or linear actuators, are found in various sizes in many types of equipment. Single-acting piston-and-cylinder units generally have a single fluid input located at a cylinder end and provide a linear force in one direction. Double-acting units generally include fluid inlets at both ends of their cylinders and provide linear force on both their extension and retraction strokes. Most hydraulic systems are "closed" whereby the actuating fluid is returned from the piston-and-cylinder units for recirculation. Pneumatic systems, on the other hand, can be "open" whereby the air, steam, etc. is released to the atmosphere after expending its energy and performing work.
Piston-and-cylinder units have heretofore been employed, for example, in petroleum recovery. Oil wells generally include submersible, reciprocating "down-hole" pumps in the production zone. These pumps are commonly actuated by pump jacks comprising pivotable walking beams connected to the pump at one end by a string of sucker rods and mounting counterweights at the other end to offset the load that must be lifted on each pumping stroke. This load, which comprises both the weight of the sucker rod string and the weight of the crude oil column in the well tubing, can be substantial in a relatively deep well. Accordingly, pump jacks often require massive counterweights. Electric motors or internal combustion engines are usually coupled to the walking beams by gear or belt-drive transmissions for providing the required rocking motion.
Although such pump jack systems have been extensively used for many years, they suffer from several disadvantages. The size, complexity and weight of a typical pump jack system, especially for a relatively deep well, add significantly to its cost. Moreover, power losses in the engines, motors and transmissions tend to reduce operating efficiencies. In fact, present electric power rates are such that the cost of operating an electric motor for a single pump jack may exceed $5,000.00 per month. Naturally, the initial and operating costs associated with pump jack systems are reflected in the cost of oil production. Furthermore, since pump jacks generally include large, moving parts that are clearly visible from a substantial distance, they tend to dramatically alter the visual aesthetics of the landscape wherever they are erected.
To overcome the aforementioned disadvantages of conventional pump jacks, hydraulic piston-and-cylinder units have heretofore been employed for actuating the down-hole pumps in oil wells. For example, a wellhead with a hydraulic pump actuator is disclosed in the Brown et al. U.S. Pat. No. 4,462,464 and includes a single-acting hydraulic piston-and-cylinder unit connected to a sucker rod string. A spool valve automatically reverses the piston-and-cylinder unit. Although the hydraulic pump actuator disclosed in this patent has certain advantages, its utility is somewhat limited because the single-acting cylinder is hydraulically driven only through its upstroke. The return or downstroke is accomplished by releasing the fluid in the cylinder lower end, whereby the piston is drawn downwardly by the weight of the sucker rod string. A double-acting actuator is often preferred for oil recovery, particularly if the down-hole pump is double-acting.
Electrically-actuated valves have also been tried on oil recovery piston-and-cylinder units. For example, one system employs upper and lower limit switches for shifting a solenoid-actuated valve at the upper and lower ends of the cylinder stroke. The valve diverts pressurized fluid from the pump to one end or the other of the hydraulic cylinder. When the cylinder reaches its uppermost or lowermost position, one of the limit switches is opened or closed whereby the solenoid-actuated valve shifts and pressurized fluid is diverted to the other cylinder end. Although this arrangement has some advantages over conventional pump jacks, a disadvantage is the dependence on the electromechanical switch and solenoid valve components. If one of these components fails, the solenoid valve may stick in one position and the system may be damaged by excessive fluid pressure accumulating in a cylinder end. Even if the system is not damaged, the failure of an important component may cause it to shut down. Downtime in oil producing rigs is generally very expensive, as are repairs since many rigs are in remote locations. Furthermore, since many rigs are unattended, a shutdown could go unnoticed until someone arrived to collect the accumulated oil and/or gas. It will be readily appreciated from the foregoing that reliability is extremely important in oil pumping systems.